Improve document focus control (#1167)

* Improve styles for #content-scroll-focus

* Create new class to manage and control document focus

* Use new focus class

* Add a check to prevent redundant .blur calls

1 files changed, 110 insertions, 0 deletions

diff --git a/ext/mixed/js/document-focus-controller.js b/ext/mixed/js/document-focus-controller.js
new file mode 100644
index 00000000..985e9f5a
--- /dev/null
+++ b/ext/mixed/js/document-focus-controller.js
@@ -0,0 +1,110 @@
+/*
+ * Copyright (C) 2020  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/**
+ * This class is used to control the document focus when a non-body element contains the main scrollbar.
+ * Web browsers will not automatically focus a custom element with the scrollbar on load, which results in
+ * keyboard shortcuts (e.g. arrow keys) not controlling page scroll. Instead, this class will manually
+ * focus a dummy element inside the main content, which gives keyboard scroll focus to that element.
+ */
+class DocumentFocusController {
+    constructor() {
+        this._contentScrollFocusElement = document.querySelector('#content-scroll-focus');
+    }
+
+    prepare() {
+        window.addEventListener('focus', this._onWindowFocus.bind(this), false);
+        this._updateFocusedElement(false);
+    }
+
+    blurElement(element) {
+        if (document.activeElement !== element) { return; }
+        element.blur();
+        this._updateFocusedElement(false);
+    }
+
+    // Private
+
+    _onWindowFocus() {
+        this._updateFocusedElement(false);
+    }
+
+    _updateFocusedElement(force) {
+        const target = this._contentScrollFocusElement;
+        if (target === null) { return; }
+
+        const {activeElement} = document;
+        if (
+            force ||
+            activeElement === null ||
+            activeElement === document.documentElement ||
+            activeElement === document.body
+        ) {
+            // Get selection
+            const selection = window.getSelection();
+            const selectionRanges1 = this._getSelectionRanges(selection);
+
+            // Note: This function will cause any selected text to be deselected on Firefox.
+            target.focus({preventScroll: true});
+
+            // Restore selection
+            const selectionRanges2 = this._getSelectionRanges(selection);
+            if (!this._areRangesSame(selectionRanges1, selectionRanges2)) {
+                this._setSelectionRanges(selection, selectionRanges1);
+            }
+        }
+    }
+
+    _getSelectionRanges(selection) {
+        const ranges = [];
+        for (let i = 0, ii = selection.rangeCount; i < ii; ++i) {
+            ranges.push(selection.getRangeAt(i));
+        }
+        return ranges;
+    }
+
+    _setSelectionRanges(selection, ranges) {
+        selection.removeAllRanges();
+        for (const range of ranges) {
+            selection.addRange(range);
+        }
+    }
+
+    _areRangesSame(ranges1, ranges2) {
+        const ii = ranges1.length;
+        if (ii !== ranges2.length) {
+            return false;
+        }
+
+        for (let i = 0; i < ii; ++i) {
+            const range1 = ranges1[i];
+            const range2 = ranges2[i];
+            try {
+                if (
+                    range1.compareBoundaryPoints(Range.START_TO_START, range2) !== 0 ||
+                    range1.compareBoundaryPoints(Range.END_TO_END, range2) !== 0
+                ) {
+                    return false;
+                }
+            } catch (e) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+}